Cem 350 control systems 9 2016
- 1. HVAC Systems Controls CEM 350 Facility Systems Design & Construction
- 2. HVAC Systems Controls - 1 Sound operation of system is directly influenced by selection and functioning of control system Temperature control systems have seen dramatic changes recently Brought about by desire to conserve energy and efficiency Most dramatic change is advent of DDC (direct digital control) and Internet (LAN, WAN – local and wide area networks)
- 3. HVAC Systems Controls - 2 Traditional controls systems: Pneumatic Electric Electronic And now DDC DDC with Internet ties
- 4. HVAC Systems Controls - 3 Traditional controls systems: Pneumatic Electric Electronic And now DDC DDC with Internet ties
- 5. HVAC Systems Controls - 4 Pneumatic control systems utilize compressed air to supply energy to control valves, damper operators etc. Pressurized air circuits are small diameter copper or plastic tubes from 1/8” dia. to 5/16” dia. Rarely exceeds ½” diameter for main air Construction tip: Protect tubing installations such as column lay-in or cable tray, conduit, etc.
- 6. HVAC Systems Controls - 5 For effective operation, we need (1): Source of clean, dry compressed air Source is usually electric air compressor with filtering removal of 99.9% of particulates 0.01 micron or smaller Filter must also remove oil introduced into air stream by compressor Smallest droplet of oil or water will render pneumatic controller with 0.005” orifice useless Remove water w/ refrigerated drier
- 7. HVAC Systems Controls - 6 For effective operation, we need (2): Pressure reducing valve (PRV) (80 psi to 20 psi – if large sysytem with long runs, consider 30-80 psi to reduce pipe sizes Air lines are copper or plastic tubing Sensing elements – measure what is to be controlled – T-stats, relays, etc. Control devices such as valve and damper operators Spring ranges – installed on pneumatic operators to provide full movement from open to closed (3 to 13 psi throttling range) so spring relaxed at 4 psi, closed at 11 psi
- 8. HVAC Systems Controls - 7 For effective operation, we need (3): Normally open valves/dampers (NO) in open position when no air pressure acts on operator. NO used when “fail-safe” condition requires full flow through valve or damper. In cold climates, heating valves are NO; warm climates, cooling coil chiller valves are NO; return air dampers typically in NO position Normally closed valves/dampers (NC) – spring pressure keeps valve in closed position if no air pressure on the operator
- 9. HVAC Systems Controls - 8 Pneumatic advantages Extremely durable – 40 year life if keep air system clean Simple operation so personnel understand Maintainability – controls readily checked with an air gauge and calibrated Very powerful and can operate large systems Generally lower costs on large projects
- 10. HVAC Systems Controls - 9 Pneumatic disadvantages Subject to oil and water contamination – oil-less air compressors not practical as they are too expensive and maintenance intensive More hardware required for initial system and for modifications – with DDC just re- program Greater deviation than DDC controls – issue with low or light load conditions Springs require calibration whereas electronic controls almost no calibration
- 11. HVAC Systems Controls - 10 Electric and Electronic Control Systems – electricity is the power source and electric wiring transmits power from controller to operator Advantages Logic panels available for similar projects or larger standard control systems (pre- packaged) Inexpensive installation for smaller projects – pneumatic systems cost- effective above 50 tons HVAC capacity Remote control of space sensors easily accomodated
- 12. HVAC Systems Controls - 11 Electric and Electronic Control Systems – Disadvantages Higher operator hardware cost Shorter service life (15-20 years) as electronics burn out due to voltage problems, short circuits, etc. Higher installed costs for larger projects – some jurisdictions require control wiring to be in conduit Operators not available for large valves
- 13. HVAC Systems Controls - 12 Direct Digital Control (DDC) – control accomplished via digital computer; employs microprocessors with software; control sequences entered into computer with software; set points by keyboard entries or touch screens Computer continuously monitors controlled variable; transmits difference in values as digital pulse that is converted to modulating signal by a transducer Transducer sends electric or pneumatic signal to controller
- 14. HVAC Systems Controls - 13 Direct Digital Control (DDC) Advantages Simplicity of changing control sequences and set points – often made without adding hardware to system Example: Increased space heat gain has caused occupant complaints; from computer VAV system can have box airflow increased, t-stat dead band reduced, cooling proportional band narrowed, system shutdown enacted later, etc. Central control point – monitor entire facility; improvement in security as
- 15. HVAC Systems Controls - 14 Direct Digital Control (DDC) Advantages Simplicity of changing control sequences and set points – often made without adding hardware to system Example: Increased space heat gain has caused occupant complaints; from computer VAV system can have box airflow increased, t-stat dead band reduced, cooling proportional band narrowed, system shutdown enacted later, etc. Central control point – monitor entire facility; improvement in security as
- 16. HVAC Systems Controls - 15 Direct Digital Control (DDC) Advantages - 2 Remote monitoring – can allow expert to “look-in” on operational issues Energy efficiency – can modify temperatures easily for occupied and unoccupied schedules Very precise control
- 17. HVAC Systems Controls - 16 DDC system components need to “talk” to each other – Disadvantages: Older components and systems difficult to “talk” to each other such as controlling automatic dampers and valves – programming required – proprietary issues between competing vendors Newer systems – more commonality